Radio receivers are susceptible to reception of spurious signals, such as that of an image frequency signal due, to the architecture and mixing products within the receiver. An image signal is a signal which, due to its relationship to the desired received signal and the local oscillator frequency, mixes to the IF (intermediate frequency) of the radio. For example, if it is desired to receive a signal having a frequency of 150 MHz and the receiver's IF frequency is 10.7 MHz, a local oscillator signal that is equal either to the sum or the difference between the desired receive frequency and the IF frequency would be utilized. In this example, the local oscillator signal would either be 160.7 MHz or 139.3 MHz in order to tune the receiver to 150 MHz. If the high side injection is used (160.7 MHz), not only would the desired frequency of 150 MHz mix to the IF frequency of 10.7 MHz, but a signal of 171.4 MHz would also be mixed to the receiver's IF frequency. Once an undesired frequency has been mixed to the radio IF frequency, it cannot be distinguished from a desired signal.
In addition to image signals, other spurious signals can disrupt the desired operation of a receiver. For example, mixing products within a radio, which are sometimes referred to as "self-quieters," can also disrupt the desired reception. Unlike an image signal, it may be possible to predict such self-quieters and a radio can be programmed on a channel-by-channel basis to utilize a local oscillator frequency that does not produce a spurious problem at a particular desired receive frequency. Such an approach is disclosed in both U.S. Pat. Nos. 4,512,035 and 4,551,856, the disclosures of which are hereby incorporated by reference. Such preprogramming is effective when it is possible to predict the actual presence of signals that may cause spurious responses due to mixing action. While this approach can be used to avoid self-quieting signals, mixing products from external signals, such as on the image frequency, cannot be so easily avoided since any superhetrodyne receiver will have an image frequency for every desired receive frequency, regardless of the chosen local oscillator frequency. A receiver operating in the field is subject to whatever RF signals happen be present at the location of the radio.